The available grip margin is a characteristic suitable to inform a driver of the level of use of a potential of his tires and to alert him to the approach of the grip limit.
International Patent Publication No. WO 03/066399 discloses a method of determining the maximum grip coefficient of a tire and the percentage of potential used, including the steps of selecting a plurality of fixed points in space (that is to say, points that are fixed in a reference frame associated with the vehicle being used), which points lie at different azimuths along the circumference in at least a sidewall of the tire, obtaining a corresponding number of measurements of circumferential distance variation (extension or contraction) at these fixed points when the tire is rolling on the ground or the road, and the measurement signals are processed so as to extract the maximum grip coefficient and the percentage of potential used.
This method allows these characteristics to be determined from distance variations measured on the rolling tire. This method requires, however, that there be a slip zone in a contact area of the tire.